JP2023131483A - Exhaust gas purification catalyst - Google Patents

Abstract

To provide an exhaust gas purification catalyst provided with a NOx occlusion layer excellent in NOx purification performance.SOLUTION: The exhaust gas purification catalyst disclosed herein includes a substrate and a catalyst layer. The catalyst layer includes a lower layer, a middle layer, and an upper layer. The lower layer contains a catalytic metal. The upper layer contains Rh. The middle layer has a front portion located on an upstream side in a flow direction of exhaust gas and a rear portion located on a downstream side. The front portion and the rear portion each contain Pt and a NOx occlusion material. The NOx occlusion material includes at least one NOx occlusion element selected from a group consisting of alkali metal elements and alkaline earth metal elements. A ratio (CA/CB) of the concentration (CA) of the NOx occlusion element contained in the front portion per 1 L of the substrate to a concentration (CB) of the NOx occlusion element contained in the rear portion per 1 L of substrate satisfies 0<CA/CB<1.SELECTED DRAWING: Figure 2

Description

The present invention relates to an exhaust gas purification catalyst. Specifically, the present invention relates to an exhaust gas purifying catalyst that includes a base material and a catalyst layer provided on the base material, and the catalyst layer includes a NOx storage layer.

Exhaust gas purification catalysts are used to remove harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO _x ) from the exhaust gas emitted from internal combustion engines such as automobile engines. There is. Three-way catalysts containing catalytic metals are widely used as exhaust gas purifying catalysts. Rh (rhodium) and Pt (platinum) and/or Pd (palladium) are often used together as catalyst metals because of their high removal efficiency of harmful components. In a typical configuration of an exhaust gas purifying catalyst, a catalyst layer containing a three-way catalyst is provided on a base material (see, for example, Patent Documents 1 to 5).

Regarding the removal of _NOx , since alkali metal components and alkaline earth metal components can temporarily store NOx, these components are used as NOx storage materials in Patent Documents 1 to 5. Among them, Patent Document 1 discloses that the catalyst layer has a three-layer structure, the middle layer of this three-layer structure is configured as a NOx storage layer containing an alkali metal component or an alkaline earth metal component as a NOx storage material, and the upper layer is further configured as a NOx storage layer containing an alkali metal component or alkaline earth metal component as a NOx storage material. It is described that excellent NOx purification performance can be obtained by forming the NOx reduction layer containing Rh.

Japanese Patent Application Publication No. 2018-143935 International Publication No. 2020/153309 Special Publication No. 2015-502855 JP2016-93760A Japanese Patent Application Publication No. 2003-24793

However, as a result of intensive study by the present inventors, it has been found that there is room for improvement in NOx purification performance in the conventional technology represented by Patent Document 1 mentioned above.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an exhaust gas purifying catalyst that includes a NOx storage layer and has excellent NOx purifying performance.

The inventors of the present invention have made the following findings as a result of intensive study. Storage of NOx by a NOx storage material containing an alkali metal component or an alkaline earth metal component occurs when NO is oxidized to NO ₂ by a catalyst metal, and this NO ₂ reacts with the alkali metal component or alkaline earth metal component. . The stored NOx is desorbed from these components by increasing the temperature. On the other hand, in order for the catalyst to exhibit high activity, it must be heated to a predetermined temperature (ie, activation temperature) or higher. Therefore, during the warm-up process of the catalyst when starting an internal combustion engine such as an engine, the NOx storage material stores _NO2 in the exhaust gas, releases _NO2 after warming up the catalyst, and the heated and activated catalyst By purifying this, the amount of NOx in the exhaust gas can be reduced.

Here, when the internal combustion engine is started, the front portion of the catalyst layer is warmed by exhaust gas, so a temperature distribution occurs within the catalyst layer. As a result, the front part of the catalyst layer releases the stored _NO2 as the temperature rises, but in the rear part of the catalyst layer, the catalyst is not heated to the activation temperature and NOx is completely purified. Can not do it. Therefore, in the above-mentioned conventional technology, NOx can be emitted as emissions.

The exhaust gas purification catalyst disclosed herein has been completed based on this knowledge. That is, the exhaust gas purification catalyst disclosed herein is disposed in an exhaust path of an internal combustion engine and is used to purify exhaust gas discharged from the internal combustion engine. Therefore, the exhaust gas purification catalyst disclosed herein is disposed in an exhaust path of an internal combustion engine and is configured to purify exhaust gas discharged from the internal combustion engine.

The exhaust gas purification catalyst disclosed herein includes a base material and a catalyst layer formed on the base material. The catalyst layer includes a lower layer located on the base material side, an upper layer located on the surface layer side, and an intermediate layer located between the lower layer and the upper layer. The lower layer contains a catalytic metal. The upper layer contains Rh. The middle layer includes a front part located upstream in the exhaust gas flow direction when disposed in the exhaust path, and a rear part located downstream of the front part in the exhaust gas flow direction. The front part and the rear part of the middle layer each contain Pt and a NOx storage material. The NOx storage material includes at least one NOx storage element selected from the group consisting of alkali metal elements and alkaline earth metal elements. The ratio ₍ C _{A /} C _B ) satisfies 0<C _A /C _B <1.

According to such a configuration, the catalyst layer having a multilayer structure is provided with a NOx storage layer having a front part on the upstream side in the flow direction of exhaust gas and a rear part on the downstream side. The concentration of the NOx storage material and the concentration of the NOx storage material in the rear portion are different from each other such that the concentration of the NOx storage material in the rear portion is higher than that in the rear portion. Therefore, with such a configuration, even if the rear portion of the catalyst layer is insufficiently warmed up at the time of starting the internal combustion engine, the NOx storage material in the rear portion, which is more abundant than the front portion of the NOx storage layer, will , can store NO ₂ released from the front part. Furthermore, since the NOx storage layer contains Pt, the NOx storage efficiency is increased.

In the exhaust gas purification catalyst disclosed herein, the Rh-containing layer is disposed above the NOx storage layer. According to such a configuration, when NO ₂ is released from the NOx storage material in the rear portion, it is possible to efficiently purify the NO ₂ . As a result, the exhaust gas purification catalyst disclosed herein can purify NOx with high efficiency while suppressing the emission of NOx as emissions when starting an internal combustion engine, and has excellent NOx purification performance.

In a preferred embodiment of the exhaust gas purifying catalyst disclosed herein, the ratio (C _A /C _B ) satisfies 0.2<C _A /C _B <0.8. According to such a configuration, the NOx purification performance becomes higher.

In a more preferable embodiment of the exhaust gas purifying catalyst disclosed herein, the ratio (C _A /C _B ) satisfies 0.4<C _A /C _B <0.7. According to such a configuration, the NOx purification performance becomes particularly high.

In a preferred embodiment of the exhaust gas purification catalyst disclosed herein, the concentration (C _A ) of the NOx storage element contained in the front portion per 1 L of the base material is more than 0 g/L and 20 g/L or less, Further, the concentration (C _B ) of the NOx storage element contained in the rear portion per 1 L of the base material is 20 g/L or more. According to such a configuration, the NOx purification performance becomes higher.

In a more preferred embodiment of the exhaust gas purifying catalyst disclosed herein, the catalytic metal contained in the lower layer is Pd. According to such a configuration, the performance of purifying harmful components of exhaust gas including HC and CO is particularly excellent.

FIG. 1 is a perspective view schematically showing an exhaust gas purifying catalyst according to an embodiment. FIG. 2 is a partial cross-sectional view of the exhaust gas purifying catalyst of FIG. 1 taken along the cylinder axis.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. Note that matters other than those specifically mentioned in this specification that are necessary for implementing the present invention can be understood as matters designed by those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the content disclosed in this specification and the common general knowledge in the field. Further, in the following drawings, members and parts that have the same function are given the same reference numerals, and overlapping explanations may be omitted or simplified. The dimensional relationships (length, width, thickness, etc.) in each figure do not necessarily reflect the actual dimensional relationships. In addition, in this specification, the notation "A to B" (A and B are arbitrary numerical values) indicating a range means not less than A but not more than B, as well as "preferably larger than A" and "preferably smaller than B." includes the meaning of

The exhaust gas purifying catalyst disclosed herein is arranged in an exhaust path of an internal combustion engine to purify exhaust gas discharged from the internal combustion engine. The exhaust gas purifying catalyst includes at least a base material and a catalyst layer formed on the base material, and the catalyst layer has a unique multilayer structure. The exhaust gas purifying catalyst disclosed herein can be placed in the exhaust system (exhaust pipe) of various internal combustion engines, particularly automobile engines, by appropriately selecting the type and shape of the base material, the design of the catalyst layer, etc. The exhaust gas purification catalyst disclosed herein will be explained below on the assumption that the internal combustion engine is a gasoline engine for an automobile, but it is intended that the exhaust gas purification catalyst disclosed herein be limited to such applications. isn't it.

<Base material>
FIG. 1 is a schematic diagram showing the structure of an exhaust gas purifying catalyst 100 according to an embodiment, and particularly shows the structure of the base material 10. As shown in FIG. In FIG. 1, arrow F indicates the flow direction of exhaust gas when placed in the exhaust path. Arrow X indicates the cylindrical axis direction of the base material 10. X1 indicates the upstream side in the exhaust gas flow direction F, and X2 indicates the downstream side in the exhaust gas flow direction F.

As the base material 10 constituting the exhaust gas purification catalyst 100, various materials and shapes conventionally used for this type of application can be employed. For example, ceramics such as cordierite, aluminum titanate, and silicon carbide are suitable as materials because they have high heat resistance. Alternatively, a substrate made of an alloy (such as stainless steel) can be used.

Regarding the form, for example, in FIG. 1, the base material 10 has a honeycomb structure having a plurality of regularly arranged cells 12 and rib walls 14 that constitute the plurality of cells 12. The cell 12 is a through hole that functions as a passage for exhaust gas. The rib wall 14 is a partition wall that partitions each cell 12. When the exhaust gas passes through the cell 12, the exhaust gas is configured to be able to come into contact with the rib wall 14. In FIG. 1, the cross-sectional shape of the cell 12 is a quadrilateral, but it may have another shape (eg, circular, triangular, hexagonal, etc.).

In FIG. 1, the outer shape of the base material 10 is cylindrical, but the outer shape of the base material 10 may be other shapes (eg, elliptical cylinder shape, polygonal cylinder shape, etc.). Although the base material 10 is a honeycomb base material in FIG. 1, it may be a base material having a foam shape, a pellet shape, or the like. The volume of the base material 10 is typically approximately 0.1 to 10L, for example 0.5 to 5L. Further, the length (total length) of the base material 10 along the cylinder axis direction X is typically approximately 10 to 500 mm, for example 50 to 300 mm. Note that in this specification, the volume of the base material 10 refers to the bulk volume including the volume of internal voids such as cells in addition to the pure volume of the base material.

In FIG. 1, the base material 10 is a straight flow type base material in which the inlet and outlet openings of the cells 12 are not closed. However, in the base material 10, the inlet side openings and outlet side openings of a large number of cells 12 are alternately closed, and exhaust gas passes from one cell (inlet side cell) through the rib wall to the adjacent cell (outlet side cell). It may be a wall-flow type (in other words, a wall-through type) base material that flows into the cell.

<Catalyst layer>
FIG. 2 schematically shows the configuration of the catalyst layer 20 included in the exhaust gas purification catalyst 100. FIG. 2 is a partial cross-sectional view of the exhaust gas purifying catalyst 100. In FIG. 2, arrows F, X, X1, and X2 have the same meanings as in FIG.

As shown in FIG. 2, the catalyst layer 20 is formed on the base material 10. Typically, catalyst layer 20 is formed on the interior surface of substrate 10. The catalyst layer 20 includes a lower layer 22 located on the base material 10 side, an upper layer 26 located on the surface layer side of the catalyst layer 20, and a middle layer 24 located between the lower layer 22 and the upper layer 26. The middle layer 24 has a front part 24a located on the upstream side X1 in the exhaust gas flow direction F, and a rear part 24b located on the downstream side X2 in the exhaust gas flow direction F than the front part 24a.

Note that the catalyst layer 20 may include layers other than the lower layer 22, the middle layer 24, and the upper layer 26 as long as the effects of the present invention are not significantly impaired. Further, in the illustrated example, the lower layer 22, the middle layer 24, and the upper layer 26 have the same thickness, but may have different thicknesses. The thickness of the lower layer 22 is not particularly limited, but is, for example, 10 μm to 30 μm, preferably 15 μm to 25 μm. The thickness of the middle layer 24 is not particularly limited, but is, for example, 40 μm to 60 μm, preferably 45 μm to 55 μm. Although not particularly limited, the thickness of the upper layer 26 is, for example, 20 μm to 40 μm, preferably 25 μm to 35 μm.

The catalyst layer 20 (that is, each of the lower layer 22, middle layer 24, and upper layer 26) contains a catalyst metal and a carrier supporting the catalyst metal.

The types of catalyst metals contained in the lower layer 22, middle layer 24, and upper layer 26 will be described later. The total content of catalyst metals in the entire catalyst layer 20 is not particularly limited, but the content of catalyst metals per 1L of volume of the base material 10 is, for example, 0.5 g/L or more, preferably 1.0 g/L. or more, and more preferably 2.0 g/L or more. On the other hand, the total content of catalytic metals in the entire catalyst layer 20 is, for example, 8.0 g/L or less, preferably 7.0 g/L or less, as the catalytic metal content per 1 L of volume of the base material 10. , more preferably 6.0 g/L or less.

The coating amount of the entire catalyst layer 20 may be appropriately determined depending on the type of the base material 10 and is not particularly limited. The coating amount of the entire catalyst layer 20 is, for example, 150 g/L to 450 g/L, preferably 200 g/L to 400 g/L, and more preferably 250 g/L to 450 g/L, per 1 L volume of the base material 10. It is 350g/L.

The coating length of the entire catalyst layer 20 (average coating length in the cylinder axis direction X of the base material 10) is, for example, 60% or more of the total length of the base material 10 (average dimension in the cylinder axis direction It may be 70% or more, 80% or more, 90% or more, or 100% of the total length of.

The coating thickness (average thickness) of the catalyst layer 20 is, for example, 3 to 300 μm, and may be 5 to 200 μm, or 10 to 100 μm.

The catalyst metal contained in the catalyst layer 20 is preferably fine particles with a sufficiently small particle size from the viewpoint of increasing the contact area with exhaust gas. The average particle size of the catalyst metal is, for example, 1 nm to 15 nm, preferably 1 nm to 10 nm, and more preferably 1 nm to 5 nm. Note that the average particle size of the catalyst metal can be obtained by acquiring a transmission electron microscope (TEM) image of the catalyst metal and determining the average value of the particle sizes of 50 or more arbitrarily selected catalyst metals in the image.

As the carrier supporting the catalytic metal, a known material used as a catalytic metal carrier of an exhaust gas purifying catalyst can be used. The carrier is typically an inorganic porous body. Examples of carriers include aluminum oxide (Al ₂ O ₃ , alumina), titanium oxide (TiO ₂ , titania), zirconium oxide (ZrO ₂ , zirconia), and silicon oxide (SiO ₂ , silica), which have an oxygen storage capacity. Materials with oxygen storage ability (OSC materials) such as ceria (CeO ₂ ) and composite oxides containing ceria; and the like. The carrier may be either a non-OSC material, an OSC material, or both.

Oxides used as non-OSC materials contain small amounts of oxides of rare earth elements such as Pr ₂ O ₃ , Nd ₂ O ₃ , La ₂ O ₃ , and Y ₂ O ₃ in order to improve heat resistance. For example, 1% by mass or more and 10% by mass or less) may be added. Since it has particularly excellent heat resistance and durability, the non-OSC material is preferably Al ₂ O ₃ , and Al ₂ O ₃ composited with La ₂ O ₃ (La ₂ O ₃ -Al ₂ O ₃ composite oxide; LA composite oxide) is more preferable.

The content of the non-OSC material in the catalyst layer 20 is not particularly limited, but is preferably 100 g/L or more and 370 g/L or less, more preferably 170 g/L or more and 300 g/L, per 1 L volume of the base material 10. /L or less.

Regarding the OSC material, examples of complex oxides containing ceria include complex oxides containing ceria and zirconia (eg, ceria-zirconia complex oxides, so-called CZ complex oxides or ZC complex oxides). When the OSC material contains zirconium oxide, thermal deterioration of cerium oxide can be suppressed, so ceria-zirconia composite oxide is preferable as the OSC material.

OSC materials contain oxides of rare earth elements, alkali metal elements, alkaline earth metal elements, transition metal elements, Si, Al, etc. for the purpose of improving properties (especially heat resistance and oxygen absorption/release properties, etc.). Good too. Examples of rare earth elements include Sc, Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and the like. Preferred rare earth element oxides are Pr ₂ O ₃ , Nd ₂ O ₃ , La ₂ O ₃ , and Y ₂ O ₃ .

When the OSC material is a composite oxide containing cerium oxide, the content of cerium oxide is preferably 10% by mass or more, more preferably 25% by mass or more, from the viewpoint of fully demonstrating its oxygen storage capacity. be. On the other hand, if the content of cerium oxide is too high, the basicity of the OSC material may become too high. Therefore, the content of cerium oxide is preferably 90% by mass or less, more preferably 75% by mass or less.

The content of the OSC material in the catalyst layer 20 is not particularly limited, but is preferably 45 g/L or more and 250 g/L or less, more preferably 80 g/L or more and 200 g/L per 1 L volume of the base material 10. L or less.

The catalyst layer 20 may further contain the above OSC material and/or the above non-OSC material in a form that does not support a catalyst metal. For example, the catalyst layer 20 may include a non-OSC material that supports a catalytic metal and an OSC material that does not support a catalytic metal.

The catalyst layer 20 may contain a binder, additives, and the like. Examples of binders include alumina sol, silica sol, and the like.

Each of the lower layer 22, middle layer 24, and upper layer 26 included in the catalyst layer 20 will be described in detail below.

<Lower layer>
Lower layer 22 contains catalytic metal. As the catalyst metal, various metal species can be used that can function as an oxidation catalyst and/or a reduction catalyst in purifying harmful components of exhaust gas. Typical examples of catalytic metals include the platinum group: rhodium (Rh), palladium (Pd), platinum (Pt), ruthenium (Ru), osmium (Os), and iridium (Ir). Further, other metal species may be used instead of or in addition to the platinum group metal. For example, metal species such as iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), silver (Ag), and gold (Au) may be used. Furthermore, an alloy of two or more of these metals may be used. As described later, since Rh, which has high reduction activity (NOx purification ability) is used in the upper layer 26, it is preferable that the catalyst metal contained in the lower layer 22 has high oxidation activity (HC and CO purification ability). . Specifically, the catalyst metal contained in the lower layer 22 is preferably at least one of Pd and Pt, and more preferably Pd. Compared to other catalyst metals, Pd has the advantage that its activity is less likely to decrease even in an oxidized state. By containing Pd in the lower layer 22, HC and CO can be removed particularly efficiently, and the exhaust gas purification catalyst 100 has particularly excellent performance in purifying harmful components of exhaust gas including HC and CO. .

The content of the catalyst metal in the lower layer 22 is not particularly limited. If the content of the catalytic metal is too low, the catalytic activity obtained by the catalytic metal may be insufficient. On the other hand, if the content of the catalytic metal is too large, the catalytic metal tends to cause grain growth and is also disadvantageous in terms of cost. Therefore, the content of the catalyst metal in the lower layer 22 is preferably 0.1 g/L to 5 g/L, more preferably It is 0.5g/L to 3g/L.

Preferably, the lower layer 22 includes an OSC material. The OSC material may be a carrier or non-carrier. More preferably, the lower layer 22 includes an OSC material and a non-OSC material. The content of the OSC material in the lower layer 22 is preferably 20 g/L to 100 g/L, more preferably 30 g/L to 80 g/L, per 1 L volume of the base material 10. Further, the content of the non-OSC material in the lower layer 22 is preferably 30 g/L to 130 g/L, more preferably 50 g/L to 100 g/L, per 1 L volume of the base material 10.

The lower layer 22 may contain auxiliary materials including alkali metal elements, alkaline earth metal elements, rare earth metal elements, transition metal elements, and the like. Auxiliary materials may be oxides, hydroxides, carbonates, nitrates, sulfates, phosphates, acetates, formates, oxalates, halides, etc. of these elements. For example, when the lower layer 22 contains an alkaline earth element (e.g., calcium (Ca), barium (Ba), etc.), poisoning of the catalyst metal (particularly the oxidation catalyst) can be suppressed by the alkaline earth element. I can do it. Further, the alkaline earth element improves the dispersibility of the catalyst metal, and can suppress sintering accompanying grain growth of the catalyst metal. Furthermore, when the lower layer 22 contains an alkaline earth element together with the OSC material, the amount of oxygen absorbed into the OSC material can be further improved in a lean atmosphere (oxygen-excess atmosphere) where the fuel is thinner than the stoichiometric air-fuel ratio. .

In this embodiment, since the middle layer 24 functions as a NOx storage layer, the lower layer 22 does not need to contain substantially NOx storage material. In the present specification, a layer substantially free of a certain component means that the content of the certain component based on the total mass of the layer is 1% by mass or less (preferably 0.5% by mass or less, more preferably 0.5% by mass or less). 1% by mass or less, more preferably 0% by mass). Preferably, the lower layer 22 is substantially free of NOx storage material, auxiliary material containing an alkali metal element, and auxiliary material containing an alkaline earth metal element.

<Middle layer>
In the catalyst layer 20, the middle layer 24 functions as a catalyst layer and a NOx storage layer.

The front part 24a and the rear part 24b of the middle layer 24 each contain Pt as a catalyst metal. Since Pt has the highest NO oxidation performance among catalyst metals, by including Pt in the middle layer 24, NO can be converted to NO ₂ in the stored form in the vicinity of the NOx storage material. Furthermore, Pt has high reactivity with paraffinic HC, and by including Pt in the middle layer 24, HC can be efficiently purified. The content of Pt in the middle layer 24 is not particularly limited. If the content of Pt is too low, the catalytic activity obtained by Pt may become insufficient. On the other hand, if the content of Pt is too high, Pt tends to cause grain growth and is also disadvantageous in terms of cost. Therefore, the content of Pt per liter of volume of the base material 10 is preferably 0.1 g/L to 5 g/L, more preferably 0.5 g/L to 3 g/L.

It is preferable that the concentration of Pt (P _A ) in the front portion 24 a of the middle layer 24 is approximately the same as the concentration of Pt (P _B ) in the rear portion 24 b of the middle layer 24 . Specifically, the ratio ( _PA / _PB ) preferably satisfies 0.5≦ _PA / _PB ≦1.5, and preferably satisfies 0.8≦ _PA / _PB ≦1.2. is more preferable, and most preferably P _A /P _B =1.0.

The front part 24a and the rear part 24b of the middle layer 24 each contain a NOx storage material containing at least one NOx storage element selected from the group consisting of alkali metals and alkaline earth metals. The type of the NOx storage material is not particularly limited as long as it contains the above elements, but typically the NOx storage material contains the at least one NOx storage element and contains carbonate or oxidation. A compound in the form of a product, or a combination thereof. Examples of alkali metals include Li, K, Cs, etc., with K being preferred among them. Examples of alkaline earth metals include Mg, Sr, Ba, etc., with Ba being particularly preferred. Particularly preferred as the NOx storage material is one compound selected from the group consisting of Ba-containing oxides and Ba-containing carbonates.

In the middle layer 24 of this embodiment, the concentration (C _B ) of the NOx storage element contained in the rear part 24b per 1 L of the base material (hereinafter also referred to as "NOx storage element concentration (C _B ) in the rear part 24b") ) is higher than the concentration (C _A ) of the NOx storage element contained in the front part 24a per liter of base material (hereinafter also referred to as "NOx storage element concentration (C _A ) of the front part 24a"). There is. Therefore, the ratio (C A /C _B ) of the NOx storage element concentration (C _A ) in the front part 24a to the NOx storage element concentration (C _B ) in the rear part 24 b _{satisfies 0<C A /} C _B <1.

When the internal combustion engine is started, the front portion of the catalyst layer 20 is warmed by exhaust gas, so a temperature distribution occurs within the catalyst layer 20. The front portion 24a of the middle layer 24 of the catalyst layer 20 releases the stored NO ₂ as the temperature rises, but in the rear portion of the catalyst layer 20, the catalyst is not sufficiently heated to the activation temperature. NOx cannot be completely purified.

Therefore, in this embodiment, the rear portion 24b of the middle layer 24 of the catalyst layer 20 contains more NOx storage material than the front portion 24a. Therefore, even if the rear portion of the catalyst layer 20 is not sufficiently warmed up at the time of starting the internal combustion engine, NOx is released from the front portion 24a due to the NOx storage material in the rear portion 24b, which is present more abundantly than in the front portion 24a. It can store _NO2 . Thereby, it is possible to suppress the release of NOx as emissions when starting the internal combustion engine. In addition, if NO ₂ cannot be stored in the front section 24a after the catalyst layer 20 has been warmed up, after fuel cut (F/C), under high SV conditions, or when the control cannot follow AFR fluctuations, etc. However, the NOx storage material in the rear part 24b, which is present more abundantly than in the front part 24a, can store _NO2 released from the front part 24a.

From the viewpoint of higher NOx purification performance, the ratio (C _A /C _B ) preferably satisfies 0.1<C _A /C _B <0.85, more preferably 0.2<C _A /C _B <0.8, more preferably 0.4<C _A /C _B <0.7.

From the viewpoint of higher NOx purification performance, the NOx storage element concentration (C _A ) in the front part 24a is more than 0 g/L and 20 g/L or less, and the above concentration (C _B ) of the NOx storage material in the rear part 24b is set. is preferably 20 g/L or more. The NOx storage element concentration (C _A ) of the front portion 24a is preferably 3 g/L or more, more preferably 5 g/L or more. The NOx storage element concentration (C _B ) in the rear portion 24b is more preferably 30 g/L or more, and more preferably 37.5 g/L or more. Note that due to manufacturing limitations and the like, the NOx storage element concentration (C _B ) in the rear portion 24b may be 40 g/L or less.

Preferably, the middle layer 24 includes an OSC material. The OSC material may be a carrier or non-carrier. More preferably, the middle layer 24 includes an OSC material and a non-OSC material. The content of the OSC material in the middle layer 24 is preferably 20 g/L to 100 g/L, more preferably 40 g/L to 80 g/L, per 1 L volume of the base material 10. Further, the content of the non-OSC material in the middle layer 24 is preferably 40 g/L to 120 g/L, more preferably 60 g/L to 100 g/L, per 1 L volume of the base material 10.

In addition, in FIG. 2, in the middle layer 24, the front part 24a and the rear part 24b are in contact with each other. However, in the middle layer 24, the front part 24a and the rear part 24b may be separated. Furthermore, for reasons such as the manufacturing method using slurry, a part of the end of the front part 24a and a part of the end of the rear part 24b may overlap.

As described above, due to reasons such as the manufacturing method using slurry, a portion of the end of the front portion 24a and a portion of the end of the rear portion 24b may overlap. Therefore, in order to avoid this overlapping part, the analysis of the NOx storage element concentration (C _A ) in the front part 24a and the NOx storage element concentration (C _B ) in the rear part 24b is performed from the end of the base material 10 to the base material 10. This may be done at a position of 30% of the length.

The coat length of the front portion 24a is preferably 30% to 70%, more preferably 40% to 60%, and even more preferably 45% to 55% of the total length of the base material 10 (ie, the average dimension in the direction of the cylinder axis). Similarly, the coat length of the rear portion 24b is preferably 30% to 70% of the total length of the base material 10, more preferably 40% to 60%, even more preferably 45% to 55%.

<Upper layer>
The upper layer 26 contains Rh as a catalytic metal. Compared to other catalytic metals, Rh has a high ability to generate H ₂ and a high ternary performance. In particular, Rh has a high NOx purification ability. Therefore, since the upper layer 26, which is closer to the surface layer than the middle layer 24, contains Rh as a catalytic metal, when the NOx stored in the middle layer 24 is released, it is possible to efficiently remove NOx.

The content of Rh in the upper layer 26 is not particularly limited. If the Rh content is too low, the catalytic activity obtained by Rh may become insufficient. On the other hand, if the Rh content is too high, Rh tends to cause grain growth and is also disadvantageous in terms of cost. Therefore, the Rh content per liter of volume of the base material 10 is preferably 0.01 g/L to 1.0 g/L, more preferably 0.05 g/L to 0.5 g/L.

Preferably, upper layer 26 includes an OSC material. The OSC material may be a carrier or non-carrier. More preferably, the upper layer 26 includes an OSC material and a non-OSC material. The content of the OSC material in the upper layer 26 is preferably 5 g/L to 50 g/L, more preferably 10 g/L to 40 g/L, per 1 L volume of the base material 10. Further, the content of the non-OSC material in the upper layer 26 is preferably 40 g/L to 120 g/L, more preferably 60 g/L to 100 g/L, per 1 L volume of the base material 10.

The upper layer 26 may contain auxiliary materials including rare earth metal elements, transition metal elements, and the like.

In this embodiment, since the middle layer 24 functions as a NOx storage layer, the upper layer 26 does not need to contain substantially NOx storage material. In one preferred embodiment, the upper layer 26 is substantially free of NOx storage material, auxiliary material containing an alkali metal element, and auxiliary material containing an alkaline earth metal element.

As described above, according to the exhaust gas purifying catalyst 100 including the catalyst layer 20 having a unique multilayer structure, NOx can be purified with high efficiency while suppressing the emission of NOx as an emission when starting an internal combustion engine. It has excellent NOx purification performance.

<<Method for manufacturing exhaust gas purification catalyst 100>>
The exhaust gas purifying catalyst 100 can be manufactured, for example, by the following method. First, a base material 10 and a catalyst layer forming slurry for forming the catalyst layer 20 are prepared. The slurry for forming a catalyst layer is, for example, a catalyst metal source (for example, a solution containing a catalyst metal as an ion) and other components (for example, a NOx storage material, a non-OSC material, an OSC material, a binder, various additives, etc.). , can be prepared by mixing in a dispersion medium. As the dispersion medium, for example, water, a mixture of water and a water-soluble organic solvent, etc. can be used. The properties of the slurry (for example, viscosity, solid content, etc.) can be appropriately determined depending on the size of the base material 10 used, the form of the cells 12 (rib walls 14), the characteristics required for the catalyst layer 20, etc. To adjust the viscosity of the slurry, it is advantageous to use cellulosic polymers such as carboxymethylcellulose (CMC), methylcellulose (MC), hydroxypropylmethylcellulose (HPMC), and hydroxyethylmethylcellulose (HEMC).

Specifically, for example, a slurry for forming a lower layer containing a catalytic metal source, a slurry for forming a middle layer containing a Pt source and a NOx storage material, and a slurry for forming an upper layer containing a Rh source are prepared. At this time, as the slurry for forming the middle layer, a slurry for forming the middle layer front part in which the concentration of alkali metal elements and alkaline earth metal elements contained in the NOx storage material is low with respect to the total solid content, and A slurry for forming a middle layer rear portion having a high concentration of alkali metal elements and alkaline earth metal elements contained in the NOx storage material is prepared.

The slurry for forming the lower layer is applied to the base material by a known coating method (eg, impregnation method, wash coating method, etc.), and after drying, it is fired to form the lower layer. Subsequently, a slurry for forming a middle layer front portion is applied from the front end of the base material to a predetermined position according to a known method, and dried. On the other hand, a slurry for forming a middle layer rear portion is applied from the rear end of the base material to a predetermined position according to a known method and dried. Then, by firing this, a middle layer is formed. Further, a slurry for forming an upper layer is applied thereto by a known coating method, and after drying, it is fired to form an upper layer. Drying conditions depend on the shape and dimensions of the substrate or carrier, but are typically about 70 to 150°C (for example, 90 to 130°C) for about 1 to 10 hours. Firing conditions are typically about 300 to 800°C (for example, 400 to 500°C) for about 1 to 4 hours. In the manner described above, the exhaust gas purifying catalyst 100 can be obtained.

≪Applications of exhaust gas purification catalyst 100≫
The exhaust gas purification catalyst 100 is suitable for use in vehicles such as cars and trucks, motorcycles and motorized bicycles, marine products such as ships, tankers, watercraft, personal watercraft, outboard motors, lawn mowers, chainsaws, It can be suitably used for purifying exhaust gas emitted from gardening products such as trimmers, leisure products such as golf carts and four-wheeled buggies, power generation equipment such as cogeneration systems, and internal combustion engines such as garbage incinerators. Among these, it can be suitably used for vehicles such as automobiles, and in particular, it can be suitably used for vehicles equipped with a gasoline engine.

Test examples related to the present invention will be described below, but the present invention is not intended to be limited to those shown in the test examples below.

<Test base material>
A cylindrical cordierite honeycomb substrate having a diameter of 118.4 mm, a total length of 114.3 mm, and a substrate capacity of 1.26 L as shown in FIG. 1 was prepared as a substrate for each Example and each Comparative Example. One end of this base material was defined as a front end (that is, an exhaust gas inflow end), and the other end was defined as a rear end (that is, an exhaust gas outflow end).

<Production of exhaust gas purification catalysts of Examples 1 to 4 and Comparative Examples 1 to 5>
A palladium nitrate aqueous solution, ZC composite oxide powder, Al ₂ O ₃ powder, and ion-exchanged water were mixed to prepare a Pd-containing slurry for the lower layer. This Pd-containing slurry for the lower layer was applied onto the above-mentioned base material over 100% of the length of the base material (dimension in the cylinder axis direction) by a wash coating method. This was dried at 90°C for 1 hour and then fired at 500°C for 1 hour to form a lower layer. The amount of Pd per unit volume of the base material was 74 g/cft (about 2.61 g/L).

A platinum nitrate aqueous solution, ZC composite oxide powder, Al ₂ O ₃ powder, barium carbonate, and ion-exchanged water were mixed to prepare a Pt-containing slurry for the middle layer. A plurality of types of Pt-containing slurries for the middle layer were prepared with different amounts of Ba. In addition, a Pt-containing slurry for the middle layer that did not contain barium carbonate was also prepared. The first Pt-containing slurry for the middle layer was applied to an area from the front end of the substrate to 50% of the length of the substrate by a wash coat method, dried at 90°C for 1 hour, and then heated at 500°C. A front part was formed by firing for 1 hour. In addition, the second middle layer Pt-containing slurry was applied to an area from the rear end of the base material to 50% of the length of the base material by a wash coat method, and after drying at 90°C for 1 hour, The rear portion was formed by firing at ℃ for 1 hour. This formed a middle layer having a front part and a rear part. The first Pt-containing slurry for middle layer and the second Pt-containing slurry for middle layer each have a Ba concentration (C _A ) in the front part and a Ba concentration (C _B ) in the rear part per 1L of base material. Those with a Ba content having the values shown in Table 1 were selected. The amount of Pt per unit volume of the base material was the same for both the front part and the rear part, and was 30 g/cft (approximately 1.06 g/L).

Rhodium nitrate aqueous solution, ZC composite oxide powder, Al ₂ O ₃ powder, and ion exchange water were mixed to prepare an Rh-containing slurry for the upper layer. This Rh-containing slurry for the upper layer was applied over 100% of the length of the base material on the middle layer formed above by a wash coating method. This was dried at 90°C for 1 hour and then fired at 500°C for 1 hour to form an upper layer. The amount of Rh per unit volume of the base material was 6 g/cft (about 0.21 g/L). In this way, exhaust gas purifying catalysts of Examples 1 to 4 and Comparative Examples 1 to 5 were obtained. The Ba concentration in the front part (C _A ) and the Ba concentration in the rear part (C _B ) were analyzed at a position 30% of the length of the base material from the front end and rear end of the base material, respectively. It was confirmed that the Ba concentration in the front part (C _A ) and the Ba concentration in the rear part (C _B ) were the values shown in Table 1.

<Preparation of exhaust gas purification catalyst of Comparative Example 6>
The above Pd-containing slurry for the lower layer was applied onto the above base material over 100% of the length of the base material by a wash coating method. This was dried at 90°C for 1 hour and then fired at 500°C for 1 hour to form a lower layer. The amount of Pd per unit volume of the base material was 74 g/cft (about 2.61 g/L).

A platinum nitrate aqueous solution, a rhodium nitrate aqueous solution, a ZC composite oxide powder, an Al ₂ O ₃ powder, barium carbonate, and ion-exchanged water were mixed to prepare a Pt and Rh-containing slurry for the upper layer. Two types of Pt and Rh containing slurries for the upper layer were prepared with different amounts of Ba. The first slurry containing Pt and Rh for the upper layer was applied to an area from the front side end of the substrate to 50% of the length of the substrate by a wash coat method, and after drying at 90°C for 1 hour, A front portion was formed by firing at ℃ for 1 hour. In addition, a second slurry containing Pt and Rh for the upper layer was applied to an area from the rear end of the base material to 50% of the length of the base material by a wash coat method, and after drying at 90°C for 1 hour. The rear portion was formed by firing at 500° C. for 1 hour. This formed an upper layer having a front part and a rear part. At this time, the Ba concentration (C _A ) in the front part and the Ba concentration (C _B ) in the rear part per 1 L of the base material were set to the values shown in Table 1. The amount of Pt per unit volume of the base material is 30 g/cft (approximately 1.06 g/L) for both the front and rear parts, and the amount of Pd per unit volume of the base material is 37 g for both the front and rear parts. /cft (approximately 1.31 g/L). In this way, an exhaust gas purifying catalyst of Comparative Example 6 was obtained.

<NOx storage amount evaluation>
The exhaust gas purifying catalysts of each Example and each Comparative Example were cooled to 100° C. or lower and then installed in an exhaust pipe of an engine bench. Exhaust gas having a λ value of 1.08 whose temperature was adjusted to 300° C. using a heat exchanger was allowed to flow into the exhaust gas purification catalyst. At this time, the NOx concentration at the position before the catalyst inflow and the NOx concentration at the position after the catalyst outflow were measured, and based on these, the NOx storage amount (mg/L) per 1 L of the base material was calculated. The results are shown in Table 1.

<NOx purification rate evaluation>
The exhaust gas purifying catalysts of each Example and each Comparative Example were cooled to 100° C. or lower and then installed in an exhaust pipe of an engine bench. Exhaust gas having a λ value of 1.08 whose temperature was adjusted to 300° C. using a heat exchanger was allowed to flow into the exhaust gas purification catalyst. At this time, the cumulative amount of NOx at the position before the inflow of the catalyst for 180 seconds and the cumulative amount of NOx after the catalyst flowed out for 180 seconds were measured, and based on these, the NOx purification rate (%) was calculated. The results are shown in Table 1.

As shown in the results in Table 1, by containing Pt in the NOx storage layer, increasing the Ba concentration in the rear part of the NOx storage layer, and arranging a layer containing Rh above the NOx storage layer, NOx storage It can be seen that the amount of NOx can be increased and that NOx can be purified with high efficiency. Therefore, from the above results, it can be seen that the exhaust gas purification catalyst disclosed herein can provide an exhaust gas purification catalyst that includes a NOx storage layer and has excellent NOx purification performance.

Although specific examples of the present invention have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technology described in the claims includes various modifications of the specific examples illustrated above,
Includes changes.

10 Base material 12 Cell 14 Rib wall 20 Catalyst layer 22 Upper layer 24 Middle layer 26 Lower layer 100 Exhaust gas purification catalyst

Claims (5)

An exhaust gas purification catalyst disposed in an exhaust path of an internal combustion engine to purify exhaust gas discharged from the internal combustion engine,
base material and
a catalyst layer formed on the base material;
Equipped with
The catalyst layer includes a lower layer located on the base material side, an upper layer located on the surface layer side, and an intermediate layer located between the lower layer and the upper layer,
The lower layer contains a catalytic metal,
The upper layer contains Rh, and the middle layer includes a front part located upstream in the exhaust gas flow direction when disposed in the exhaust path, and a front part located downstream of the front part in the exhaust gas flow direction. a rear part;
The front part and the rear part of the middle layer each contain Pt and a NOx storage material,
The NOx storage material includes at least one NOx storage element selected from the group consisting of alkali metal elements and alkaline earth metal elements,
The ratio ₍ C _{A /} C _B ) satisfies 0<C _A /C _B <1,
Catalyst for exhaust gas purification. The exhaust gas purifying catalyst according to claim 1, wherein the ratio (C _A /C _B ) satisfies 0.2<C _A /C _B <0.8. The exhaust gas purifying catalyst according to claim 2, wherein the ratio (C _A /C _B ) satisfies 0.4<C _A /C _B <0.7. The concentration (C _A ) of the NOx storage element contained in the front part per 1 L of the base material is more than 0 g/L and 20 g/L or less, and the NOx storage element contained in the rear part per 1 L of the base material. The catalyst for exhaust gas purification according to any one of claims 1 to 3, wherein the concentration (C _B ) of is 20 g/L or more. The exhaust gas purifying catalyst according to any one of claims 1 to 4, wherein the catalytic metal contained in the lower layer is Pd.

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